NIFI-4962 FlattenJson processor add unexpected backslash after flatten

Signed-off-by: Pierre Villard <pierre.villard.fr@gmail.com>

This closes #2538.

nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/FlattenJson.java

@@ -20,6 +20,7 @@ package org.apache.nifi.processors.standard;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.github.wnameless.json.flattener.FlattenMode;
 import com.github.wnameless.json.flattener.JsonFlattener;
+import org.apache.commons.text.StringEscapeUtils;
 import org.apache.nifi.annotation.behavior.SideEffectFree;
 import org.apache.nifi.annotation.documentation.CapabilityDescription;
 import org.apache.nifi.annotation.documentation.Tags;
@@ -160,6 +161,7 @@ public class FlattenJson extends AbstractProcessor {
             final String flattened = new JsonFlattener(raw)
                     .withFlattenMode(flattenMode)
                     .withSeparator(separator.charAt(0))
+                    .withStringEscapePolicy(() -> StringEscapeUtils.ESCAPE_JAVA)
                     .flatten();
 
             flowFile = session.write(flowFile, os -> os.write(flattened.getBytes()));

nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/test/groovy/org/apache/nifi/processors/standard/TestFlattenJson.groovy

@@ -172,4 +172,23 @@ class TestFlattenJson {
             Assert.assertEquals("Separator not applied.", "one", parsed["first.second.third.0"])
         }
     }
+
+    @Test
+    void testFlattenSlash() {
+        def testRunner = TestRunners.newTestRunner(FlattenJson.class)
+        def json = prettyPrint(toJson([
+                first: [
+                        second: [
+                                third: [
+                                        "http://localhost/value1", "http://localhost/value2"
+                                ]
+                        ]
+                ]
+        ]))
+
+        testRunner.setProperty(FlattenJson.FLATTEN_MODE, FlattenJson.FLATTEN_MODE_NORMAL)
+        baseTest(testRunner, json,2) { parsed ->
+            Assert.assertEquals("Separator not applied.", "http://localhost/value1", parsed["first.second.third[0]"])
+        }
+    }
 }

nifi-nar-bundles/nifi-standard-bundle/pom.xml

@@ -329,7 +329,7 @@
             <dependency>
                 <groupId>com.github.wnameless</groupId>
                 <artifactId>json-flattener</artifactId>
-                <version>0.4.1</version>
+                <version>0.5.0</version>
             </dependency>
             <dependency>
                 <groupId>org.apache.bval</groupId>